As is known in the art, microprismatic retroreflective film generally consists of a plastic film containing many microscopic cube corner retroreflective elements (also known as either microprismatic retroreflective elements or microprisms). These retroreflective elements are generally a trihedral structure having three mutually substantially perpendicular lateral faces. The three lateral faces intersect at a single point, or apex, which is opposite of a base triangle, which outlines the base of the trihedral. The optical axis of a cube corner element is defined as the axis which extends from the apex of the cube corner retroreflector to trisect the base triangle. These cube corner retroreflective elements operate to return impinging light towards its source. Light will enter each cube corner retroreflective element through the base triangle and will then be reflected from each of the three lateral faces to return towards its source.
Reflection from the three lateral faces occurs either through specular reflection or total internal reflection. With specular reflection, the cube corner retroreflective elements are coated with a reflective material, such as either aluminum or silver, as is the case with metalized microprismatic retroreflective sheeting. With total internal reflection, the cube corner retroreflective elements are not coated with a reflective material, but instead are governed by Snell's Law where any light impinging on one of the lateral faces will pass through the face unless it strikes the face at an angle less than its critical angle, in which case the light will be reflected.
There are four main classifications of microprisms, depending upon how the optical axis is positioned relative to one or more of the lateral faces of the microprisms. These four categories are equilateral microprisms, forward-tilted microprisms, backward-tilted microprisms, and scalene microprisms. A more detailed discussion of these classifications is provided below.
In practice, microprismatic retroreflective elements are interconnected in arrays of microprismatic elements throughout a roll of retroreflective sheeting. There are thousands of individual microprisms per a square-inch of sheeting, and all of these retroreflective elements function together to provide the overall retroreflective performance properties of the sheeting. Further, as known in the art, the microprisms within these arrays generally occur in matched pairs, which is a by-product of how microprisms are often designed and manufactured.
There are many applications for retroreflective sheeting, including traffic signs, vehicle number plates, contour marking tape for trucks and other heavy vehicles, reflective vehicle graphics, construction work zone barricades or delineators, safety vests, railroad crossings, and many other applications. In many of these applications, the retroreflective sheeting is aligned either in a vertical or horizontal position. For example, contour markings on a heavy truck trailer are typically applied horizontally along the sides of a trailer. Similarly, reflective sheeting is often applied to several horizontal boards to create a construction work zone barricade. Further, school buses often have retroreflective tape outlining the perimeter of emergency exits.
The performance of retroreflective sheeting is largely characterized by two retroreflective performance parameters: entrance angularity and observation angularity. Entrance angle is defined as the angle at which light impinges on the surface of the retroreflective sheeting. A zero degree entrance angle is perpendicular to the face of the sheeting. In most instances, the retroreflective properties of microprismatic retroreflective sheeting are greatest at small entrance angles and decrease as the entrance angle increases.
Observation angle is defined as the angle between the source light and the detector as light strikes the face of the retroreflective sheeting and is retroreflected back towards the source. For example, for a truck, the observation angle is the angle between the truck's headlight (the light source), the retroreflective object (such as a traffic sign), and the truck driver's eye (the detector). Observation angles are typically quite small, between 0.1 and 2.0 degrees. For example, when a retroreflective traffic sign is viewed from about 700 feet from an automobile, an observation angle of about 0.2 degrees occurs. Generally speaking, the closer the source light (such as a vehicle's headlight) is to the retroreflective object, the larger the observation angle. Similarly, larger observation angles will exist when a retroreflective object is viewed from a heavy truck instead of a small car because the truck driver is typically further elevated above the truck's headlights, and therefore, larger angles exist between the driver's eyes and the headlights.
Hoopman, U.S. Pat. No. 4,588,258 and Szczech, U.S Pat. No. 5,138,488 both teach how improved entrance angularity can be achieved in both a horizontal and vertical plane by incorporating forward-tilted microprisms into the sheeting. However, the improved entrance angularity between the horizontal and vertical planes is not symmetrical.
Nestegard, et al., U.S. Pat. Nos. 5,706,132 and 5,936,770 teach how tiling can be used to balance the improved entrance angularity of forward-tilted prisms by creating at least two different arrays of microprisms arranged perpendicular to each other.
Smith, et al., U.S. Pat. No. 5,764,413 outlines the principles of tiling for backward-tilted microprisms to optimize the entrance angle performance in both vertical and horizontal planes.
Smith, et al., U.S. Pat. Nos. 5,812,315 and 5,822,121 teach how entrance angularity can be improved in both vertical and horizontal planes by rotating either backwards-tilted or scalene microprisms with respect to the edge of the sheeting.
Nilsen, et al., U.S. Pat. Nos. 6,036,322, 6,457,835, and 6,877,866 teach how a more uniform distribution of light can be achieved through tiling of four arrays of microprisms at 0°, 90°, 180° and 270°, especially for backward-tilted prisms.
Appeldorn, et al., U.S. Pat. No. 4,775,219 and Couzin, et al., U.S. Pat. No. 6,984,047 both teach how to improve observation angularity by intentionally creating dihedral angle ruling errors.
As mentioned above, there are generally two different product constructions for microprismatic retroreflective sheeting available in the market. The first construction is encapsulated microprismatic sheeting, where the cube corner retroreflective elements are encapsulated within a cellular structure. With this construction, the microprismatic elements operate by the principles of total internal reflection. The second construction is metalized microprismatic sheeting, where the cube corner retroreflective elements have been coated with a reflective coating, such as aluminum or silver. With metalized retroreflective sheeting, the microprisms operate through principles of specular reflection.